1. Field of the Invention
The invention generally relates to radio transmitters, and in particular relates to an apparatus and method for band-pass filtering in the transmitter feedback loop.
2. Description of the Related Art
Standards relating to radio communications such as TETRA (Terrestrial Trunked RAdio), UMTS (Universal Mobile Telecommunications System) and EDGE (Enhanced Data Rates for GSM Evolution) generally require a high degree of linearity in transmitter equipment to reduce noise between closely spaced radio channels. Linearization of power amplifiers in transmitter equipment has been extensively researched and many techniques such as Cartesian loop, Polar loop, Envelope Elimination and Restoration, LINC (LInear amplification using Nonlinear Components) and CALLUM (combined analog locked loop universal modulator) have been produced.
Linearity and bandwidth are traded off in these techniques, with high linearity being possible over narrow bandwidth and moderate linearity over a broader bandwidth. Most techniques also trade linearity for efficiency. Power amplifiers used in radio transmitters are more efficient when operated at higher power but then have lower linearity, particularly near their peak power ratings. These techniques are less satisfactory for mobile communications that require both high linearity and also high efficiency for longer battery life and lower weight.
The Cartesian loop technique involves negative feedback applied to a baseband input signal having in-phase and quadrature components. The feedback signal is a measure of distortion introduced in the forward path of the loop, primarily by the amplifier, and is subtracted from the input signal in real time. This modifies the input signal with an error signal that tends to cancel the distortion at the output of the amplifier.
In order to obtain a stable feedback system, it is required that the feedback quadrature signals are approximately 180 degrees shifted relative to the incoming quadrature signals when the feedback loop is closed. Due to the unknown and variable phase shift generated by the feedback loop under antenna loading, this condition is not always fulfilled. The incoming and feedback quadrature signals are usually brought into the required relative phase with each other with the aid of a compensating phase rotation in the feedback loop. A common method to determine the phase rotation generated by the feedback loop is to open the loop and to measure the incoming in-phase and quadrature signals (I/Q), and the feedback quadrature signals. The measured values are analog-to-digital converted, and the phase error is calculated. Thereafter, a voltage-controlled phase rotator is regulated by the converted digital values to apply a phase shift in Cartesian loop systems to counter radio frequency (RF) delays around the feedback loop.
To ensure the accuracy of the phase rotation, existing Cartesian loop circuits provide noise filtering to remove the additional noise and distortion introduced by the Cartesian loop. However, Cartesian feedback loop band-pass filter designs can be complicated because quadrature generation circuits require strong filtering of input signal harmonic levels to preserve quadrature balance. Moreover, different channels usually require different phase shifts and different optimum settings, further complicating the filter designs. To achieve adequate attenuation of the input signal harmonic levels, while still supporting the large fractional bandwidths required in such systems, band-pass filters for phase rotators have traditionally increasing the number of polls and, correspondingly, the complexity and cost of the band-pass filters. What is needed is a band-pass filter integrated into the Cartesian feedback loop that filters the input signal harmonic levels at large fractional bandwidths, while remaining a low-complexity filter design.